Callimachus, Hymn 4 to Delos 275 ff (trans. Mair) (Greek poet C3rd B.C.) :

"Thou [Delos] art famed as the most holy of islands, nurse of Apollon’s youth. On thee treads not Enyo nor Haides nor the horses of Ares; but every year tithes of first-fruits are sent to thee : to thee all cities lead up choirs, both those cities which have cast their lots toward the East and those toward the West and those in the South, and the Hyperboreans (peoples which have their homes above the northern shore), a very long-lived race. These first bring thee cornstalks and holy sheaves of corn-ears, which the Pelasgians of Dodona [i.e. the famous oracle of Zeus] . . . first receive, as these offerings enter their country from afar. Next they come to the Holy town and mountains of the Malian land; and thence they sail across to the goodly Lelantian plain of the Abantes [i.e. the island of Euboia]; and then not long is the voyage from Euboia, since thy havens are nigh thereto. The first to bring thee these offerings from the fair-haired Arimaspoi were Oupis and Loxo and happy Hekaerge, daughters of Boreas, and those who then were the best of the young men. And they returned no home again, but a happy fate was theirs, and they shall never be without their glory. Verily the girls of Delos, when the sweet-sounded marriage hymn affrights the maidens’ quarters, bring offerings of their maiden hair to the maidens, while the boys offer to the young men the first harvest of the down upon their cheeks."

Callimachus, Fragment 187 (from Clement Protrept. 25) (trans. Trypanis) (Greek poet C3rd B.C.) :“Phoibos [Apollon] visits the Hyperborean sacrifices of asses.”

Callimachus, Fragment 187 (from Scholiast on Pindar’s Pythian 10. 49) :“Fat sacrifices of asses delight Phoibos.”

Callimachus, Fragment 215 (from Scholiast on Apollonius Rhodius 4. 284) :“They [the Hyperboreans] send [offerings to Apollon at Delos] from the Rhipaion Mountains.”